In recent years, LED lamps have been widely used as a superb energy-conscious replacement for incandescent light bulbs. The LED module used in an LED lamp is often, for example, a gallium nitride (GaN) blue LED used in combination with YAG yellow phosphor. Such an LED module produces blue light from the blue LED, which excites the yellow phosphor particle, which in turn produce yellow light. Combined, the blue light and the yellow light result in white light.
Typically, in the field of lighting, objects viewed under a light source should preferably appear to have natural coloring. In other words, a better color rendering index is valued highly. Various conventional technologies have been proposed for enhancing the color rendering index of LED lamps. For example, Patent Literature 1 proposes mixing neodymium oxide (Nd2O3) particles into resin or the like for a filter element attached to the LED lamp as a means of improving average color rendering index Ra. Also, Patent Literature 2, which describes a fluorescent lamp, proposes forming a glass thin-film filter that incorporates neodymium ions (Nd3+) in the space between a glass tube and a phosphor layer, as a means of improving the color rendering index.
Japanese Industrial Standard Z8726 defines methods for quantitatively evaluating the color rendering index under a light source by using color rendering indices indicating color reproduction fidelity in comparison to a reference light source.